The present invention relates to an art that reduces the consumption current on an electronic circuit.
Reduction of consumed current is strongly desired for electronic circuits, such as DC-DC converters used in personal digital assistants.
FIG. 4 is a schematic diagram of an overcurrent detection circuit used in, for example, a DC-DC converter, and is disclosed in JP-A-2004-140423.
In FIG. 4, M0 and M1 are both n-channel MOSFETs (metal oxide field-effective transistors), wherein M0 is the main field effect transistor (FET) in an output stage of a DC-DC converter and is used in driving a load ZL. The drain current Id0 of M0 flows from the power line VD to ground, i.e., the reference potential, through the load ZL and M0.
M1 is an FET functioning as a reference load, wherein Id1 is the drain current based on a reference current source Iref, and Iref is a constant-current source from the power line VDD to the ground. In this circuit, Id1 is held equal to Iref, and the mirror ratio of M0 and M1 is assumed to be M.
In this circuit, M0 and M1 have a common gate potential VG equal to each other. In this case, when the drain current Id0 is greater than M×Id1 (also M×Iref), as in the case when Vd0, the drain potential of M0, is greater than Vd1, the drain voltage of M1, the drain current of M0, i.e., Id0, is determined to be an overcurrent.
A comparator COMP compares M0's drain voltage, Vd0, with a reference voltage Vd1 produced by M1. When the relationship Vd0<Vd1 changes into a relationship Vd0> Vd1, such as when the M0 drain current Id0 exceeds M×Iref, the output of comparator COMP changes from “H” to “L”, to indicate the overcurrent status.
In the overcurrent detecting circuit of FIG. 4, the overcurrent determining threshold, M×Iref, is determined by the relative characteristics of M0 and M1. In the case that M0 and M1 having matching characteristics, for example, when formed on a same semiconductor substrate, the determining threshold is enhanced in the stability against disturbances, such as temperature change.
FIG. 5 is a partial schematic of an internal circuit configuration of the comparator COMP shown in FIG. 4.
As shown in FIG. 5, the comparator COMP is configured with a differential amplifier section having n-channel MOSFETs M11 and M12 and p-channel MOSFETs M13 and M14. A bias current is supplied by a constant-current source Ib to the differential amplifier section.
FIG. 5 omits an output amplifier section provided in the stage following the differential amplifier section in the comparator COMP.
In addition, M13 and M14 form a differential pair, wherein the gate terminal of M13 is a non-inverted input and M14 is an inverting input. The gate terminal of M13 is fed with a reference voltage Vd1, produced by a reference load M1 and the gate terminal of M14 is fed with a detection voltage Vd0 that has been detected by M0 (FIG. 4). In addition, the current drawn from the power line VDD due to the constant current source Ib branches into two parts and serves as an input to the respective source terminals, M13 and M14.
The respective gate terminals of M11 and M12, and the drain terminal of M11 are connected to the drain terminal of M13. Accordingly, M11 and M12 form a current mirror that makes the M12 drain current equal in magnitude to the M11 drain current. The drain terminal of M12 is connected to the drain terminal of M14, whose connection point provides an output Vout of the differential amplifier section. The respective source terminals of M11 and M12 are connected to ground.
Still referring to FIG. 5, the differential amplifier section formed by M11, M12, M13 and M14 amplifies the difference in the potentials between the respective gate input terminals of M13, M14, the output Vout being the amplified difference. The output, Vout, is further amplified in an output amplifier section in the following stage (not shown) and becomes the output of comparator COMP.
In the overcurrent detecting circuit of FIG. 4, if the mirror ration M can be increased, even if a current supplied from the reference current source Iref is reduced, the same determining threshold M×Iref is obtained, thus reducing the current consumption of the overcurrent detecting circuit.
However, the value M has an upper limit. For example, to provide an overcurrent determining threshold of 500 mA for an M0 having a semiconductor channel width of M0 of 50000 μm, even when the channel width of M1 is reduced to 5 μm, thereby providing M=1000, a reference current Iref as great as 50 μA is required. The value of Iref accounts for a comparatively large percentage of the current, for example, 200-300 μA, consumed by the entire control circuit for a DC-DC converter commonly used in a personal digital assistant.
Meanwhile, in the FIG. 4 circuit, the comparator COMP, for comparing the magnitude between the drain potentials Vd0 and Vd1, requires a certain degree of consumption current (e.g. 50 μA) in order to obtain a sufficient response speed.